Visual sensitivity is measured by determining how much light or how much change is needed for a subject to detect a stimulus. The necessary amount of light or change is affected by many factors, including the size, color, and temporal mode of presentation of the stimulus. In addition, sensitivity is a function of the location of the stimulus on the retina, and state of adaptation of the subject. Visual sensitivity declines with age and as a result of disease processes. Therefore, in order to determine the effectiveness of various efforts to reduce this decline, methods of measuring different types of visual sensitivity, including both light and dark-adapted sensitivity, as well as the rates of change of adaptation states, have been developed. Visual sensitivity measurements are also useful in determining the state of health of the retina and the neural pathways of the visual system. A further important use of the measurements of visual sensitivity is to determine the optical density of components of the eye.
One ocular component of importance is the yellow macular pigment, which is an accumulation of the carotenoids lutein and zeaxanthin in the foveal region of the retina. The spectral properties of this yellow pigment afford protection to the eye from short wavelength blue light. In addition, these two carotenoids are associated with retention of better visual sensitivity by older subjects. Therefore, assessing the in vivo concentration of macular pigment can be a valuable means of determining visual health. Lower macular pigment levels may also be associated with a higher risk of contracting age-related macular degeneration. Determination of in vivo macular pigment levels is possible, however, only if the amount of yellow pigment can be measured in situ.
The ability to measure the macular pigment concentration in situ results from the spectral characteristics of the yellow pigment itself: blue light, at a wavelength of 460 nm, is strongly absorbed by the yellow pigment whereas green light, at a wavelength of 560 nm, is not absorbed. The subject's own retina is used to perform both a baseline measurement and a foveal sensitivity measurement to obtain quantitative data about the amount of macular pigment in the fovea of the eye. Physical, in vivo, determinations of yellow macular pigment have been made using elaborate optical systems with many components [59]. Alternatively, the desired measurements can be obtained by psychophysical determinations.
Psychophysics is a hybrid field of science encompassing study of the perceptual (psychological) responses of subjects to sensory stimuli that are carefully controlled and measured by the methods of physics. For the visual system, light is the sensory stimulus. When light is absorbed by the photoreceptors of the retina, a neural response is generated that results in detection of the stimulus by the subject. The ocular tissues can be considered a set of optical filters through which the light must pass. By careful choice of the wavelength and the mode of presentation of the stimulus, the subject's responses can reveal the optical properties of the ocular filters. In essence, the visual system of the subjects is used as an intricate photosensory apparatus to measure the optical density spectrum of components of human ocular tissues.
The advantage of psychophysical methodology is that it is noninvasive, yet extremely sensitive. An individual can be followed over time, and the same tissue can be repeatedly measured. Psychophysical methods are therefore well suited to studying the effects of nutritional and environmental influences on the aging process. Nevertheless, psychophysical methodology has distinct limitations. The measurements usually require relatively prolonged testing by a skilled examiner. They can not be used with very young children, infirm or gravely handicapped individuals, or persons with dense cataracts that severely obscure vision. Furthermore, the subject must clearly understand the task and must be able to give reliable reports of sensory experience. This reliance on the participation and performance of the subject always introduces a risk of misunderstanding, with the need for careful cross-checking by the experimenter.
In the past, psychophysical methods have been used primarily in laboratories that have specialized and complex optical systems. Psychophysical determinations are most commonly made with a multi-channel Maxwellian-view optical bench set-up that requires operation by scientists with extensive experience with complex optical systems (e.g., [1, 8]). In Maxwellian view, the stimulus is imaged on the retina by an external lens that has a focal point centered in the subject's pupil. The beam of light that enters the eye is very small so that it only traverses the center of the subject's lens and does not intersect the subject's pupil. Aligning the subject's eye with the external Maxwellian lens, however, requires that the position of the head be precisely controlled. It is customary to make a "bite bar" with an impression of the subject's teeth so that when the subject rests the upper jaw on the bite bar, the head can be positioned to align the eye with the optical system. Once this is done, stimuli can be presented with the techniques of classical optics and the absorption of light by the macular pigment determined. One method of measuring this absorption uses flicker photometry. In this technique, two test colors are turned on and off at about 12 Hz (12 times per second). As the intensity of one of the test colors changes so that the test colors approach the same brightness, the perception of flicker diminishes. In using this method to measure macular pigment, the subject adjusts a blue light, alternating with a green light, until minimum or zero flicker is perceived. This setting, compared to a baseline, is a measure of visual sensitivity to blue light and, thus, the in vivo concentration of the blue-absorbing macular pigment.
A second major ocular component whose optical properties can be determined by visual sensitivity measurements is the lens of the eye. For these measurements, the dark-adapted visual sensitivity dominated by the rod photoreceptors can also be determined psychophysically. If the ocular media were perfectly transparent, the visual sensitivity of the observer would be determined solely by the visual pigment of the rod photoreceptors, rhodopsin [34]. Therefore, the deviation of the observer's sensitivity from the rhodopsin absorption spectrum provides a measure of the density of the ocular media, and most of this density is known to be due to the lens [35, 36]. Because the density spectrum of the lens is well known [36], the whole spectral curve can be estimated by measuring visual sensitivity at two or more wavelengths, which can be the same ones used to determine macular pigment.
Many disease states, as well as losses of function due to aging, appear to be preferentially evidenced by changes in the sensitivity of the pathway receiving inputs from the cone photoreceptors that absorb short-wavelength light, called the S-cone pathway. By alternating between two wavelengths that excite the S-cones but not the other cone types, it is possible to preferentially excite the S-cone pathway and measure its sensitivity. Attempts to improve S-cone pathway sensitivity by nutritional or other means can then be monitored to determine their effects and optimize the outcome of the interventions.
Thus, psychophysical determinations of ocular components can be useful for the assessment of a wide array of health-related conditions. The availability of an apparatus that was simpler to use for these determinations than the currently available complex systems would be desirable.